Nucleic acid affinity columns

ABSTRACT

This invention provides nucleic acid affinity matrices that bear a large number of different nucleic acid affinity ligands allowing the simultaneous selection and removal of a large number of preselected nucleic acids from the sample. Methods of producing such affinity matrices are also provided. In general the methods involve the steps of a) providing a nucleic acid amplification template array comprising a surface to which are attached at least 50 oligonucleotides having different nucleic acid sequences, and wherein each different oligonucleotide is localized in a predetermined region of said surface, the density of said oligonucleotides is greater than about 60 different oligonucleotides per 1 cm 2 , and all of said different oligonucleotides have an identical terminal 3′ nucleic acid sequence and an identical terminal 5′ nucleic acid sequence. b) amplifying said multiplicity of oligonucleotides to provide a pool of amplified nucleic acids; and c) attaching the pool of nucleic acids to a solid support.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 09/910,223, filed Jul. 20, 2001. now U.S. Pat. No. 6,440,677,which is a divisional of U.S. patent application Ser. No. 09/429,521,filed Oct. 28, 1999, now U.S. Pat. No. 6,280,950, which is a divisionalof U.S. patent application Ser. No. 08/815,395, filed Mar. 10, 1997, nowU.S. Pat. No. 6,013,440, which derives priority from provisionalapplication USSN 60/013,231, filed Mar. 11, 1996, all of which areincorporated herein by reference in their its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to matrices for conductingnucleic acid affinity chromatography. More specifically, the presentinvention relates to methods of preparing affinity chromatographymatrices that bind a plurality of different preselected nucleic acids.The matrices, for example, can bind to substantially every known nucleicacid message in a sample.

BACKGROUND OF THE INVENTION

Affinity chromatography has become a valuable tool for separatingbiological materials from fluid (typically aqueous) media. Examplesinclude biologically active molecules such as small ligands, proteins,nucleic acids, enzymes, etc.

The basic principle of affinity chromatography involves immobilizationof a binding moiety (e.g., a ligand) to an insoluble support. Theimmobilized binding moiety can then be used to selectively adsorb, e.g.,from a fluid medium, the target component(s) (e.g. an enzyme) with whichthe binding moiety specifically interacts thereby forming a bindingmoiety/target complex. Elution of the adsorbed component can then beachieved by any one of a number of procedures which result indisassociation of the complex. Thus the specific biologic properties ofbiological macromolecules can be exploited for purification. The processcan be used to isolate specific substances such as enzymes, hormones,specific proteins, inhibitors, antigens, antibodies, etc. on the basisof the biologic specific interactions with immobilized ligands.

Nucleic acid affinity chromatography is based on the tendency ofcomplementary, single-stranded nucleic acids to form a double-strandedor duplex structure through complementary base pairing. A nucleic acid(either DNA or RNA) can easily be attached to a solid substrate (matrix)where it acts as an immobilized ligand that interacts with and formsduplexes with complementary nucleic acids present in a solutioncontacted to the immobilized ligand. Unbound components can be washedaway from the bound complex to either provide a solution lacking thetarget molecules bound to the affinity column, or to provide theisolated target molecules themselves. The nucleic acids captured in ahybrid duplex can be separated and released from the affinity matrix bydenaturation either through heat, adjustment of salt concentration, orthe use of a destabilizing agent such as formamide, Tween-20, or sodiumdodecyl sulfate (SDS).

Hybridization (the formation of duplex structure) between two nucleicacid sequences is highly sequence dependent. Sequences have the greatestaffinity with each other where, for every purine in one sequence(nucleic acid) there exists a corresponding pyrimidine in the othernucleic acid and vice versa. This sequence dependency confers exquisitespecificity on hybridization reactions and permits the preparation ofaffinity columns that are highly selective for particular target nucleicacids.

Affinity columns (matrices) are typically used either to isolate asingle nucleic acid typically by providing a single species of affinityligand. Alternatively, affinity columns bearing a single affinity ligand(e.g. oligo dt columns) have been used to isolate a multiplicity ofnucleic acids where the nucleic acids all share a common sequence (e.g.a polyA).

SUMMARY OF THE INVENTION

This invention provides pools (solutions) of nucleic acids, and nucleicacid affinity matrices that bear a large number of different nucleicacid affinity ligands allowing the simultaneous selection and blockingor removal of a large number of different preselected nucleic acids froma sample. This invention additionally provides methods and devices forthe preparation of such affinity matrices.

In one embodiment, this invention provides a method of making a nucleicacid pool (solution of nucleic acids) comprising a plurality ofdifferent nucleic acids. The method includes first, providing a nucleicacid amplification template array comprising a surface to which areattached at least 20 oligonucleotides having different predetermined(known) nucleic acid sequences; and second, amplifying the multiplicityof oligonucleotides at least about 10 fold to provide the nucleic acidpool. The oligoncleotides, or subsequences thereof, preferably encode“capture probes” which can be incorported into an affinity matrix. In apreferred embodiment, each different oligonucleotide is localized in apredetermined region of the surface, the density of the oligonucleotidesis preferably greater than about 60 different oligonucleotides per 1cm², and the different oligonucleotides preferably have an identicalterminal 3′ nucleic acid subsequence and an identical terminal 5′nucleic acid subsequence. The 3′ and 5′ nucleic acid subsequences can bethe same as each other or can differ in length and/or nucleotidesequence. The 3′ and 5′ subsequences preferably flank “unique” centralsubsequences encoding the capture probes.

The method can further involve attaching the pool of nucleic acids to asolid support to form a nucleic acid affinity matrix.

The template nucleic acids comprising the amplification template can besynthesized entirely using light-directed polymer synthesis or channelmethods. Alternatively the template nucleic acids can be synthesizedusing a combination of methods. For example, in one embodiment, the 3′segments (subsequences) of the template nucleic acids can be synthesizedusing standard phosphotriester (e.g., phosphoramidite) chemistry. Amiddle (unique) portion of the template nucleic acids can then besynthesized using light-directed polymer synthesis ormechanically-directed synthesis methods. Finally, the 5′ segments(subsequences) of the template nucleic acids can be synthesized usingphosphotriester chemistry.

The template nucleic acids can be amplified using any nucleic acidamplification method (e.g. polymerase chain reaction, ligase chainreaction, transcription amplification, etc.). In a preferred embodiment,amplification is by PCR. The template nucleic acids can be released intosolution prior to the amplification (e.g. by cleavage of a linkerjoining the template nucleic acids to the substrate) thereby allowingthe amplification to be performed in solution. Alternatively, and in apreferred embodiment, the amplification is performed without releasingthe template nucleic acids from the substrate.

In a preferred embodiment, the amplification templates include primerbinding regions (e.g. 3′ and 5′ subsequences flanking the regionencoding the capture probe). Preferred amplification templates includeidentical 3′ and 5′ primers. The primer binding regions of theamplification template oligonucleotides, and hence the correspondingcomplementary PCR primers, preferably range in length from about 4 toabout 30 nucleotides. The primer binding regions can be identical toeach other or can differ in nucleotide sequence and/or in length.

In a particularly preferred embodiment, the region of the amplificationtemplates encoding the capture probes (the non-identical portion of theamplification template(s)) ranges in length from about 6 to about 50nucleotides. Where it is desired to remove the primer binding regions,they can include a recognition site of a nuclease to facilitatecleavage. In a particularly preferred embodiment, the thermal meltingpoints of the template nucleic acid sequences encoding the captureprobes with their complementary sequences varies by less than about 20°C.

In another embodiment, this invention provides for nucleic acidamplification template arrays for practice of the above-describedmethod. In a preferred embodiment, the template arrays comprise apredetermined multiplicity of at least 20 oligonucleotides havingdifferent nucleic acid sequences. Each different oligonucleotide ispreferably localized in a predetermined region of said surface. Thedensity of the oligonucleotides is preferably greater than about 60different oligonucleotides per 1 cm², and the different oligonucleotideshave identical terminal 3′ nucleic acid subsequences (e.g., primerbinding region) and identical terminal 5′ nucleic acid subsequences(e.g., primer binding region), The 3′ and 5′ subsequences can beidentical to each other or differ in length and/or nucleotide sequence.The subsequences (primer binding regions) of the oligonucleotides, andhence the corresponding complementary PCR primers, preferably range inlength from about 4 to about 30 nucleotides.

The region of the template nucleic acids comprising the amplificationtemplate array encoding the capture probe (the “unique” non-terminalsubsequence) preferably ranges in length from about 6 to about 50nucleotides. Where it is desired to remove the primer binding regions,the 3′ and/or 5′ subsequences can include a recognition site of anuclease to facilitate cleavage. In a particularly preferred embodiment,the thermal melting points of the template nucleic acid sequencesencoding the capture probes with their complementary sequences varies byless than about 20° C.

In another embodiment this invention provides an affinity matrix thatremoves substantially all known nucleic acid messages in a sample andmethods of making such an affinity matrix. In a preferred embodiment,the affinity matrix comprises a multiplicity of at least 20 differentpredetermined oligonucleotides where, for each nucleic acid message,there exists in the affinity matrix an oligonucleotide complementary tothe nucleic acid message or a subsequence thereof. The matrix, however,does not include every possible oligonucleotide having the same lengthas the predetermined oligonucleotides. The oligonucleotides can beselected such that the affinity matrix includes fewer than 80% of thetotal number of possible nucleotides, preferably fewer than 60% of thetotal number of possible nucleotides, more preferably fewer than 40% tothe total number of possible oligonucleotides, and most preferably lessthan about 30% or even 20% or even 10% or even 5% of the total possiblenumber of oligonucleotides having the same length as the predeterminedoligonucleotides. Oligonucleotides comprising preferred nucleic acidmatrices range in length from about 6 to about 50 nucleotides.

Oligonucleotides for inclusion in such affinity matrices can be selectedas described herein by the steps of i) determining an allowable T_(m)interval, ii) determining a mismatch T_(m) threshold; iii) identifyingall nucleic acid sequences complementary to a known message whose T_(m)to said message is within the allowable T_(m) interval; iv) determiningthe likelihood of each of the nucleic acid sequences complementary tothe known message also occurring in an unknown message; v) sorting thesequences in order of likelihood with the least likely sequence first toproduce a sorted sequence list; vi) selecting the first nucleic acidsequence in the list whose T_(m) to all other known messages in thesample is below the mismatch T_(m); vii) repeating step vi) until adesired number of nucleic acids that specifically hybridize, understringent conditions, to the known message are obtained; and viii)repeating steps iii) through vii) until at least one nucleic acidsequence that hybridizes specifically under stringent conditions to eachknown nucleic acid message is selected. Step (vi) can further compriseselecting the probe that additionally has a T_(m) to all alreadyselected nucleic acids below the mismatch T_(m).

In one embodiment the allowable T_(m) interval ranges from about 30° C.to about 80° C. In another preferred embodiment, the mismatch T_(m) isat least 5° C. lower than the allowable T_(m) interval. The likelihoodcan be determined by calculating the probability of occurrence of eachof the nucleic acid sequences of step (iii) in a calculated nucleic acidprobability distribution. The oligonucleotides can be produced byamplification from a nucleic acid amplification template array asdescribed above and further herein. Further details on the selection ofoligonucleotides in the matrix are provided herein.

In still yet another embodiment, this invention provides a nucleic acidaffinity matrix that binds to N previously unknown nucleic acid messagesand methods of making such nucleic acid matrices. The method involvesthe steps of first providing a multiplicity of at least N differentpredetermined oligonucleotides each oligonucleotide complementary to anunknown nucleic acid message predicted to be present in a nucleic acidsample or complementary to a subsequence of the unknown nucleic acidmessage; and second, attaching the nucleic acids to a solid support. Theoligonucleotides can be selected by: i) providing a list of all possibleoligonucleotides of length K; ii) deleting from the list all of theoligonucleotides that hybridize to known nucleic acid messages; iii)calculating a probability of occurrence in a nucleic acid distributionof each of the probes remaining in the list; iv) sorting the list fromhighest probability to lowest probability; v) selecting the highestprobability oligonucleotide for inclusion in the affinity matrix; andvi) repeating steps (iii) through (v) until N oligonucleotides areselected. The selection of step (vi) can further comprise recalculatingthe probability on the condition that probability distribution containsno nucleic acids complementary to those oligonucleotides alreadyselected. Selection step (v) can further include selecting an allowableT_(m) interval and selecting the highest probability oligonucleotidewhose T_(m) lies within the allowable T_(m) interval. Theoligonucleotides can be amplified from the nucleic acid amplificationtemplate arrays described above. In a particularly preferred embodiment,the oligonucleotides are attached to a solid support (e.g. glass beads)by a covalent linkage to a biotin which is joined to a streptavidinwhich is covalently joined to the solid support.

Finally, in still yet another embodiment, this invention provides amethod to enrich a nucleic acid sample for previously unknown expressedRNA sequences. The method includes the steps of: i) providing anaffinity matrix having at least one oligonucleotide complementary toeach known expressed RNA present in a sample; ii) hybridizing RNA froman undifferentiated control cell and differentiated or activated testcell respectively to the affinity matrix thereby removing knownexpressed RNAs from the control cell and the differentiated or activatedtest cell; iii) reverse transcribing the RNA from each of the controlcell and the differentiated or activated test cell to produce a cDNA,wherein the reverse transcription adds a polymerase chain reactionprimer binding region to the cDNAs from the differentiated or activatedtest cell; iv) combining the cDNAs from the differentiated or activatedtest cell with the cDNA from the control cell such that there is morecDNA from the control cell than cDNA from the differentiated oractivated test cell; v) amplifying the mixture of cDNAs using primerscomplementary to the primer binding regions such that the amplificationresults in an enrichment of nucleic acid sequences transcribed in thedifferentiated or activated test cell at a significantly higher levelthan in the control cell. In a preferred embodiment, ratio of cDNA fromthe control cell to cDNA from the test cell, in step (iv) is at leastabout a 5:1, more preferably at least about 10:1, most preferably atleast about 20:1.

Definitions

As used herein, an “oligonucleotide” refers to a single stranded nucleicacid having a length greater than 2 nucleotides, more preferably greaterthan about 5 nucleotides, and most preferably greater than about 10, 15,20, or 50 oligonucleotides. The oligonucleotides of this invention canrange in length up to about 1000 nucleotides, but preferred lengthsrange up to a maximum of about 500, more preferably up to about 250nucleotides, and most preferably up to about 150 nucleotides (bases). Anoligonucleotide can include natural (i.e., a, G, C, T or U) or modifiedbases (i.e., 7-deazaguanosine, inosine, etc.). In addition, the bases inan oligonucleotide can be joined by a linkage other than aphosphodiester bond, so long as it does not interfere with hybridizationof the oligonucleotide. Thus, oligonucleotides can be peptide nucleicacids in which one or more of the constituent bases are joined bypeptide bonds rather than phosphodiester linkages.

The term nucleic acid “affinity matrix”, as used herein, refers to asolid support or gel to which is attached a multiplicity of differentoligonucleotides. It is recognized that a nucleic acid template array,itself can act as an affinity matrix. However, in a preferredembodiment, where greater loading (binding) capacity is preferred, theaffinity matrix is fabricated using nucleic acids amplified from thetemplate array. Preferred matrix materials do not interfere withsubsequent hybridization of attached oligonucleotides. Suitable matrixmaterials include, but are not limited to paper, glasses, ceramics,metals, metalloids, polacryloylmorpholide, various plastics and plasticcopolymers such as Nylon™, Teflon™, polyethylene, polypropylene,poly(4-methylbutene), polystyrene, polystyrene, polystyrene/latex,polymethacrylate, poly(ethylene terephthalate), rayon, nylon, poly(vinylbutyrate), polyvinylidene difluoride (PVDF), silicones,polyformaldehyde, cellulose, cellulose acetate, nitrocellulose, andcontrolled-pore glass (Controlled Pore Glass, Inc., Fairfield, N.J.),aerogels (see, e.g., Ruben et al., J. Materials Science 27, 4341-4349(1992); Rao et al., J. Material. Science 28, 3021 (1993); Back et al.,J. Phys. D. Appl. Phys. 22, 730-734 (1989); Kim & Jang, J. Am. Ceram.Soc. 74, 1987-92 (1991) and the like, and other materials generallyknown to be suitable for use in affinity columns (e.g. HPLC columns).

The term “target nucleic acid” refers to a nucleic acid (often derivedfrom a biological sample), to which the oligonucleotide probe isdesigned to specifically hybridize. It is the target nucleic acid(s)that the affinity matrices of this invention are designed to capture(bind). The target nucleic acid(s) have sequences that are complementaryto the nucleic acid sequence of the oligonucleotide affinity ligand inthe affinity matrix. The term target nucleic acid may refer to thespecific subsequence of a larger nucleic acid to which oligonucleotideis complementary or to the overall sequence (e.g., gene, cDNA or mRNA)that it is desired to capture. The difference in usage will be apparentfrom context.

The term “subsequence” refers to a partial sequence of a longer nucleicacid.

The term “affinity ligand” as used herein refers to a molecule presentin the affinity matrix that specifically binds to, and thereby captures,a target molecule. Oligonucleotides are preferred affinity ligands inthe affinity matrices of this invention.

The terms “nucleic acid template” or “template”, as used herein, referto a nucleic acid that acts as a template for a nucleic acidamplification method. Nucleic acid templates of the present inventionserve as templates for the amplification of nucleic acid poolscomprising capture probes that are used either in solution or bound to asolid support to provide nucleic acid affinity matrices. Preferrednucleic acid templates additionally include primer binding regions tofacilitate amplification. A particularly preferred nucleic acid templatecomprises a unique sequence (subsequence) that encodes the nucleic acidcapture probe, flanked on the 5′ and 3′ ends by subsequences that act asprimer binding regions.

The term “nucleic acid pool” as used herein, refers to a heterogenouscollection of nucleic acids. For example, a nucleic acid pool cancomprises at least 100, 1000, or 10,000 different nucleic acids. Thenucleic acids within a pool often lack an imposed relationship. Forexample, a pool can be formed from nucleic acids lacking substantialsequence identity with each other (e.g., less than 50% or 75% sequenceidentity) to each other. Sequence identity is determined betweenoptimally aligned sequences by standard algorithms such as GAP, BESTFIT,FASTA, and TFASTA (Wisconsin Genetics Software Package Release 7.0,Genetics Computer Group, 575 Science Dr., Madison, Wis.). Nucleic acidswithin the pool typically range in size from 5-100 bases, preferably,10-50 bases. Typically the nucleic acid pools are prepared byamplification of a heterogenous collection of template nucleic acids(e.g., as found in a template array).

The term “blocking reagent”, when used herein in reference to a nucleicacid pool, refers to a pool or solution of one or more nucleic acidsthat specifically bind to preselected target sequences. The duplexesthus formed are typically incapable of further hybridization.

The term “template array” or “amplification template array” refers to acollection of oligonucleotides that acts as a templates for simultaneousamplification of a collection of nucleic acids. Preferred templatearrays are used in the fabrication of affinity ligands for incorporationinto an affinity matrix.

The terms “nucleic acid” or “nucleic acid molecule” refer to adeoxyribonucleotide or ribonucleotide polymer in either single-ordouble-stranded form, and unless otherwise limited, would encompassknown analogs of natural nucleotides that can function in a similarmanner as naturally occurring nucleotides.

The phrase “nucleic acid message”, as used herein refers to a nucleicacid or subsequence thereof that is transcribed when a gene isactivated. Thus, nucleic acid messages typically include mRNAs andsubsequences thereof. However, nucleic acid messages are used herein torefer to nucleic acids indicative of the presence, absence, or amount ofsuch transcribed sequences. Thus, nucleic acid messages also includenucleic acids derived from such transcripts including, but not limitedto cDNA, cRNA, amplification products, and so forth.

The phrase “hybridizing specifically to”, refers to the binding,duplexing, or hybridizing of a molecule only to a particular nucleotidesequence under stringent conditions when that sequence is present in acomplex mixture (e.g., total cellular) DNA or RNA. The term “stringentconditions” refers to conditions under which a probe will hybridize toits target subsequence, but to no other sequences. Stringent conditionsare sequence-dependent and will be different in different circumstances.Longer sequences hybridize specifically at higher temperatures.Generally, stringent conditions are selected to be about 5° C. lowerthan the thermal melting point (T_(m)) for the specific sequence (orabout 5° C. lower than the sequence with the highest melting point for agroup of sequences) at a defined ionic strength and pH. The T_(m) is thetemperature (under defined ionic strength, pH, and nucleic acidconcentration) at which half the duplex molecules (i.e. half the basepairs) are dissociated, or the point where the denaturation rate equalsthe renaturation rate under given conditions. Typically, stringentconditions will be those in which the salt concentration is less thanabout 0.01 to 1.0 M Na ion concentration (or other salts) at pH 7.0 to8.3 and the temperature is at least about 30° C. for short probes (e.g.,16 to 50 nucleotides). Stringent conditions may also be achieved withthe addition of destabilizing agents such as formamide.

The term “capture probe”, as used herein, refers to a nucleic acid thatis complementary to a target nucleic acid. The capture probe, whenincorporated into an affinity matrix acts as an affinity ligand that canspecifically hybridize to and thereby capture its respective targetnucleic acid. It is recognized that capture probes can also exist insolution (e.g. in nucleic acid pools) where they may act as blockingprobes or where they can be subsequently bound to a solid support toproduce an affinity matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a preferred method of making the affinity matrix ofthis invention. Briefly, a nucleic acid template array bearing aplurality of amplification templates for affinity ligands where eachtemplate comprises an affinity ligand (probe) sequence flanked by PCRprimer binding sites (primers a* and B*) is synthesized (e.g. synthesisusing light-directed coupling or mechanically-directed coupling). Theaffinity templates are amplified via polymerase chain reaction (PCR)using a biotinylated primer. The resulting biotinylated amplificationproduct is then purified via HPLC (e.g. Using a mono Q column,Pharmacia, Piscataway, N.J., USA) and then combined with streptavidincoated beads. The resulting affinity matrix is then packed into acolumn.

DETAILED DESCRIPTION

I. Amplified Nucleic Acid Pools and Affinity Matrices

This invention provides a method of preparing pools of nucleic acidsand, by attaching the pool(s) of nucleic acids to a solid support, amethod of preparing nucleic acid affinity matrices. Both the pool ofnucleic acids and the affinity matrices comprise a number of differentpreselected nucleic acids that act as affinity ligands. Unlike affinitycolumns found in the prior art, the nucleic acid pools and affinitymatrices of this invention bear a large number of different preselectednucleic acid affinity ligands. The nucleic acid pools and affinitymatrices of this invention can thus be used to simultaneously bind toand capture a large number of different nucleic acids and therebyprovide a sample with either reduced representation of those nucleicacids or conversely, where the selected nucleic acids are retrieved, asample enriched for the selected collection of nucleic acids. Both theamplified pools of nucleic acids and the affinity matrices of thisinvention have a large number of uses which are discussed below.

A) Amplified Nucleic Acid Pools.

The nucleic acid pools of this invention are particularly useful whenused as a blocking reagents (blocking probes). A blocking reagent is areagent that reduces or prevents the hybridization of one or morenucleic acids to particular components of a sample. For example, whereit is desired to detect a particular (target) nucleic acid that isexpressed at low levels in a nucleic acid sample, selective eliminationof other nucleic acids that are present at high levels in the sample canimprove detection and isolation of the target sequence. In this case, anucleic acid pool containing nucleic acids complementary to the nucleicacids it is desired to block in the sample can be hybridized to thesample. The nucleic acid pool (blocking reagent) will hybridize tocomplementary sequences in the sample, form stable hybrid duplexes, andthereby prevent interaction (e.g., nonspecific binding) of the blockednucleic acids with the capture sequence. Thus, for example, where geneproducts, such as actin or GADPH, are typically overexpressed in anucleic acid sample, the nucleic acid pool will be prepared containingnucleic acids complementary to those (e.g., actin and GADPH) RNAs. Whenthe blocking reagent is combined (hybridized) to the sample, the GADPHand actin RNAs will hybridize with the blocking reagent and theirparticipation in other reactions will be reduced or eliminated.

In another embodiment, the nucleic acid pools can be used to providecontrol nucleic acid mixtures for use as test standards for evaluationand quality control of various nucleic acid detection and/or isolationmethods. Nucleic acid pools can be prepared containing nucleic acidsthat differ from each other in only one, two, three, or more particularnucleotides. The nucleic acid pool can then be used as a standard samplefor evaluating the ability of a method (e.g. a particular HPLC column)to isolate and/or distinguish between the various nucleic acidcomponents of the pool. In this way the ability of a device or method todetect, isolate, and/or discriminate particular nucleic acids from apool of closely related nucleic acids can be evaluated.

B) Affinity Matrices

The affinity matrices of this invention are also useful in a widevariety of contexts. For example, where it is desired to quantify mRNAexpression levels of particular genes in a complex nucleic acid sample(e.g., total mRNA) (see. e.g., PCT/US97/01603, filed Jan. 22, 1997,currently in the national stages. incorporated by reference in itsentirety for all purposes) it is often desirable to eliminate nucleicacids produced by genes that are constitutively overexpressed andthereby tend to mask gene products expressed at characteristically lowerlevels. Thus, in one embodiment, the affinity matrix can be used toremove a number of preselected gene products (e.g., acrin, GAPDH, etc.).This is accomplished by providing an affinity matdx bearing nucleic acidaffinity ligands complementary to the gene praducts (e.g., mRNAS ornucleic acids derived therefrom) or to subsequences thereof.Hybridization of the nucleic acid sample to the affinity matrix willresult in duplex formation between the affinity ligands and their targetnucleic acids. Upon elution of the sample from the affinity matrix, thematrix will retain the duplexes nucleic acids leaving a sample depletedof the overexpressed target nucleic acids.

The affinity matrix can also be used to identify unknown mRNAs or cDNAsin a sample. Where the affinity matrix contains nucleic acidscomplementary to every known gene (e.g., in a cDNA library, DNA reversetranscribed from an mRNA, mRNA used directly or amplified, orpolymerized from a DNA template) in a sample, capture of the knownnucleic acids by the affinity matrix leaves a sample enriched for thosenucleic acid sequences that are unknown. In effect, the affinity matrixis used to perform a subtractive hybridization to isolate unknownnucleic acid sequences. The remaining “unknown” sequences can then bepurified and sequenced according to standard methods.

The affinity matrix can also be used to capture (isolate) and therebypurify unknown nucleic acid sequences. For example, an affinity matrixcan be prepared that contains nucleic acid (affinity ligands) that arecomplementary to sequences not previously identified, or not previouslyknown to be expressed in a particular nucleic acid sample. The sample isthen hybridized to the affinity matrix and those sequences that areretained on the affinity matrix are “unknown” nucleic acids. Theretained nucleic acids can be eluted from the matrix (e.g. at increasedtemperature, increased destabilizing agent concentration, or decreasedsalt) and the nucleic acids can then be sequenced according to standardmethods.

Similarly, the affinity matrix can be used to efficiently capture(isolate) a number of known nucleic acid sequences. Again, the matrix isprepared bearing nucleic acids complementary to those nucleic acids itis desired to isolate. The sample is contacted to the matrix underconditions where the complementary nucleic acid sequences hybridize tothe affinity ligands in the matrix. The non-hybridized material iswashed off the matrix leaving the desired sequences bound. The hybridduplexes are then denatured providing a pool of the isolated nucleicacids. The different nucleic acids in the pool can be subsequentlyseparated according to standard methods (e.g. gel electrophoresis).

As indicated above the affinity matrices can be used to selectivelyremove nucleic acids from virtually any sample containing nucleic acids(e.g., in a cDNA library, DNA reverse transcribed from an mRNA, mRNAused directly or amplified, or polymerized from a DNA template, and soforth). The nucleic acids adhering to the column can be removed bywashing with a low salt concentration buffer, a buffer containing adestabilizing agent such as formamide, or by elevating the columntemperature.

In one particularly preferred embodiment, the affinity matrix can beused in a method to enrich a sample for unknown RNA sequences (e.g.expressed sequence tags (ESTs)). The method involves first providing anaffinity matrix bearing a library of oligonucleotide probes specific toknown RNA (e.g., EST) sequences. Then, RNA from undifferentiated and/orunactivated cells and RNA from differentiated or activated orpathological (e.g., transformed) or otherwise having a differentmetabolic state are separately hybridized against the affinity matricesto provide two pools of RNAs lacking the known RNA sequences.

The RNAs from the differentiated (or activated, etc.) cells are reversetranscribed to produce cDNAs bearing cleavable PCR priming regions.(See, e.g., Van Gelder, et al., Proc. Natl. Acad. Sci. USA, 87:1663-1667 (1990) and Eberwine et al. Proc. Natl. Acad. Sci. USA, 89:3010-3014). Similarly, the RNAs from the undifferentiated cells arereverse transcribed to make cDNAs without PCR priming sites. The tworesulting pools of cDNAs are then combined with the cDNAs from theundifferentiated cells in great excess (e.g. at least 10 fold excess).At this high concentration, during PCR the cDNAs from theundifferentiated cells will hybridize with complementary sequences fromthe differentiated cells and form stable duplexes thereby preventingamplification of the corresponding sequences. Conversely, those nucleicacids unique to the differentiated cells are differentially amplified.Thus, the PCR effectively performs a subtractive hybridization resultingin a nucleic acid sample enriched for unknown ESTs that arecharacteristic of the activated or differentiated state.

II. Preparation of the Affinity Matrix

Methods of preparation of the affinity matrices of this invention areillustrated in FIG. 1. The methods generally involve first identifyingthe nucleic acids (capture probes) it is desired to include in theaffinity matrix. Once the capture probes have been identified,corresponding nucleic acid templates can be designed from which thecapture probes can be amplified. The nucleic acid templates are allattached to a solid support thereby forming a “template array” (see FIG.1, “DNA chip”). In a preferred embodiment, each template nucleic acid islocated in a particular preselected region on the solid support. Thus,for example, the DNA chip of FIG. 1 shows an array of “rectangles” whereeach rectangle contains a different template nucleic acid.

The template nucleic acids are amplified en mass (e.g., via PCR) toproduce a population of different nucleic acids (see FIG. 1). The numberof molecules of each species of nucleic acid in the population will besubstantially greater than the number of template molecules used toamplify that species. The amplified nucleic acids are then attached to asolid support (e.g. glass or plastic beads) to form an affinity matrix.The affinity matrix can be arranged or packaged into a variety of forms.In one preferred affinity matrix the matrix material is glass beadshaving attached capture probes and the beads are packed into a column tothereby produce an affinity column (see FIG. 1).

The method of preparation of affinity matrices can be generallysummarized in the following steps:

1) Nucleic acids (capture probes) to be included in the affinity matrixare identified;

2) A template array is provided that bears a plurality of differentnucleic acids where each different nucleic acid is capable of acting asan amplification template to amplify one of the capture probes;

3) The nucleic acid templates in the template array are amplified toprovide a population of capture probes where the number of molecules ofeach capture probe is substantially greater than the number ofcorresponding template molecules from which the capture probe specieswas amplified; and

4) The capture probes are then attached to a solid support (matrixmaterial) to thereby produce an affinity matrix.

Each of these steps is discussed in detail below.

1) Identification/selection of Nucleic Acids to Include in the AffinityMatrix

As indicated above, the first step in the claimed method of preparingeither a free solution containing a multiplicity of preselectedoligonucleotides or an affinity matrix of this invention involvesdetermining which oligonucleotides to include in the solution or matrix.Once the desired oligonucleotides are identified, an amplificationtemplate for each oligonucleotide is designed and the collection ofamplification templates form an amplification template array.

One of skill in the art will appreciate that amplification templates canhave either the same nucleotide sequence as the desired amplifiedoligonucleotides or can be complementary to those oligonucleotides. Thesense of the dominant amplified strand can be controlled by usingunequal amounts of primer so that the primer for the undesired strand iseffectively rate limiting during the amplification step. Methods ofdetermining appropriate template sense and primer ratios are well knownto those of skill in the art (see, e.g., PCR Protocols: a Guide toMethods and Applications, Innis et al., eds. Academic Press, Inc. N.Y.1990). For convenience, when discussing template construction, templatesequences capable of amplifying the desired oligonucleotides (captureprobes) are discussed as though they have the same sequence as theamplified product, recognizing that the actual corresponding templatesequence can be the same or complementary to the ultimate amplifiedproduct.

In addition to the template sequence encoding the capture probe, thetemplate nucleic acid can include additional (ancillary) sequences tofacilitate amplification and purification of the capture probe. Thus,the design of nucleic acid templates can involve both the selection ofthe capture probe component of the template and the design and inclusionof ancillary sequences. Each of these components is described separatelybelow.

a) Identification/selection of Capture Probe Component of AmplificationTemplate

i) Arbitrary Preselected Oligonucleotides

One of skill in the art will appreciate that virtually anyoligonucleotide can be included in the affinity matrix. In a preferredembodiment, however, oligonucleotides are selected that arecomplementary to a sequence or subsequence of the nucleic acid(s) it isdesired to bind to the affinity matrix. In addition, captureoligonucleotide sequences are selected to minimize self complementaritywhich may result in hairpin formation, or other secondary structure thatmay interfere with hybridization. In addition, the oligonucleotides canincorporate various base substitutions to reduce secondary structure(e.g. inosine, 7-deazaguanosine, etc.).

In one embodiment, the oligonucleotide affinity ligands are selected tobind and thus remove nucleic acids (e.g. mRNAs) that arecharacteristically overexpressed and thus tend to mask the expression ofother nucleic acids that are typically expressed at lower levels. Suchoverexpressed nucleic acids include, but are not limited to, the common“housekeeping” genes such as actin, GAPDH, and other well knownconstitutively expressed genes.

In another embodiment, the affinity matrix oligonucleotides are selectedto hybridize to one or more target nucleic acids it is desired to purifyfrom a sample of biological molecules. Thus, for example, where it isdesired to isolate a particular set of nucleic acids from a biologicalsample, the affinity matrix is provided containing oligonucleotidescomplementary to sequences or subsequences of the nucleic acids it isdesired to isolate.

ii) Selecting Oligonucleotide Probes to Bind all Known Sequences

In still yet another embodiment, the affinity matrix nucleic acids canbe selected to bind to, and thus remove, substantially every known mRNA,cDNA, expressed sequence tag (EST), or other known nucleic acid in asample, and thus provide a nucleic acid sample enriched for unknownsequences. In a preferred embodiment, oligonucleotide “probes” designedfor such a selection are chosen to have a relatively uniform meltingpoint (T_(m)). The probes are also preferably selected to have a lowprobability of hybridizing to an unknown message and thus have a T_(m)above a characteristic threshold which is the melting point of the mostsimilar non-selected nucleic acids expected to be in the sample.

A probe selection process embodying the above-identified criteria can beformalized in the following steps:

1) An allowable T_(m) interval for the probes is selected.

2) A T_(m) threshold is selected.

3) For each known nucleic acid sequence that is to be captured by theaffinity matrix the following steps are performed:

a) All oligonucleotide probes of a preselected length complementary tothe sequence and whose T_(m) falls within the allowable T_(m) intervalare determined.

b) The likelihood of each oligonucleotide probe being a present in anunknown message is calculated;

c) The oligonucleotide probes are then sorted by likelihood with theleast likely probe listed first; and finally

d) The first probe in the list whose T_(m) to all other known messagesin the sample is below the mismatch T_(m) is selected for inclusion inthe affinity matrix.

As indicated above, the allowable T_(m) interval is arbitrary and chosenlargely for convenience. Of course, one of skill will appreciate that anarrower T_(m) will produce fewer suitable probes, while a broader T_(m)will make optimization of amplification and hybridization conditionsmore difficult. Similarly, a suitable mismatch T_(m) is also arbitrary.Of course, the mismatch T_(m) must be lower than the T_(m) intervalotherwise the probes comprising the affinity matrix will be unable tospecifically discriminate between target nucleic acids and non-targetnucleic acids having similar, but not identical, sequences.

The allowable T_(m) interval (i.e. the T_(m) of the selected probestheir complementary sequences) typically spans a maximum range of about30° C., preferably a maximum range of about 20° C., more preferably amaximum range of about 10° C. and most preferably a maximum range ofabout 5° C. Allowable T_(m) intervals range from about 30° C. to about80° C., more preferably from about 35° C. to about 70° C., morepreferably from about 40° C. to about 60° C., and most preferably about45° C. to about 55° C. As indicated above, the mismatch T_(m) is lowerthan the low side of the allowable T_(m) interval, typically at leastabout 5° C. lower, preferably at least about 10° C. lower, morepreferably at least about 15° C. lower and most preferably at leastabout 20° C. lower than the allowable T_(m) interval.

As indicated below (in section (2)) above, the oligonucleotides can havea preselected length ranging from 2 up to about 1000 nucleotides, morepreferably from about 6 to about 150 nucleotides, most preferably fromabout 6 to about 50 nucleotides.

Methods of calculating thermal melting points (T_(m)) of two nucleicacids (e.g. an oligonucleotide and its complement) are well known tothose of skill in the art. Detailed calculations are provided forexample in Chapter 2 of Laboratory Techniques in Biochemistry andMolecular Biology, Vol. 24: Hybridization With Nucleic Acid Probes, P.Tijssen, ed. Elsevier, N.Y., (1993)). (See, also Sambrook, et al.,Molecular Cloning: a Laboratory Manual, 2nd Ed., Vols. 1-3, Cold SpringHarbor Laboratory (1989)), Methods in Enzymology, Vol. 152: Guide toMolecular Cloning Techniques, Berger and Kimmel, eds., San Diego:Academic Press, Inc. (1987), or Current Protocols in Molecular Biology,Ausubel, et al. eds., Greene Publishing and Wiley-Interscience, New York(1987)).

In general, the likelihood of an oligonucleotide probe being present inan unknown message is determined by calculating a nucleic acidprobability distribution for the particular genome(s) or nucleic acidcollection(s) of interest. The probability (likelihood) of occurrence ofthe oligonucleotide probe in that distribution is then determined.

For example, a simple distribution representing the human genome is thefrequency of occurrence of each of the bases; a, C, G and T in the humangenome. Since the human genome is not yet fully sequenced, the completefrequency distribution can be estimated by extrapolation from a sampledistribution. Thus, for example, the base composition of the humangenome can be estimated by selecting all of the human genome sequenceslisted in GenBank. The total number of each nucleotide a, G, C, and Tgiven as N_(A), N_(C), N_(G), and N_(T), respectively can then be easilytotaled. The probability of each nucleic acid (p(X) where x is a, G, C,or T) can then be determined as:${p(X)} = \frac{N_{X}}{N_{A} + N_{C} + N_{G} + N_{T}}$

The likelihood of an oligonucleotide O of length Y where theoligonucleotide is represented as

O=X ₁ −X ₂ −X ₃ . . . −X _(y)

where X₁ is the nucleotide at position 1, X₂ is the nucleotide atposition 2, and so forth, occurring in the given distribution is thencalculated as

p(O)=p(X ₁)p(X ₂)p(X ₃) . . . p(X _(y))

In a nucleic acid population where the frequency of occurrence of a, G,C and T is 0.3, 0.3, 0.2 and 0.2, respectively, the probability (p(O))of an oligonucleotide having the sequence

AAGATT

is

p(O)=p(A)p(A)p(G)p(A)p(T)p(T)=(0.3)(0.3)(0.3)(0.3)(0.2)(0.2)=0.000324

Such a distribution of course reflects only base composition and lackssequence information. A probability distribution incorporating sequenceinformation can be generated by calculating the marginal probabilitydistribution for a particular base given a certain combination ofpreceding bases.

Thus, for example, the probability distribution of each dinucleotide canbe calculated as follows:$\left. {{p\left( {AA} \right.}{XA}} \right) = \frac{N_{AA}}{N_{AA} + N_{GA} + N_{CA} + N_{TA}}$

where p(AA|XA) is the probability of occurrence of the dinucleotide AAgiven a preceding a, and N_(AA), N_(GA), N_(CA) and N_(TA) are thenumbers of occurrence of the dinucleotides AA, GA, CA and TA,respectively. Similarly, the probability of occurrence of thedinucleotide AT given a preceding C is calculated as:$\left. {{p\left( {AT} \right.}{XC}} \right) = \frac{N_{AT}}{N_{AC} + N_{GC} + N_{CC} + N_{TC}}$

where N_(AC), N_(GC), N_(CC) and N_(TC) are the numbers of occurrence ofthe dinucleotides AC, GC, CC and TC, respectively. Other dinucleotideprobabilities can be determined in the same manner.

The likelihood of an oligonucleotide occurring in the distribution isthen calculated as the product of the marginal probability (frequency ofoccurrence) of its respective dinucleotides given the preceding base.Thus, for example the likelihood of oligonucleotide O occurring in thedistribution where O is ACGTTACC is

p(O)=p(AC|XC)p(CG|XG)p(GT|XT)p(TT|XT)p(TA|XA)p(AC|XC)p(CC|XC)p(C)

One of skill will readily appreciate that similar distributions can becalculated for trinucleotides, tetranucleotides and so forth to anyarbitrary length. Each higher order distribution incorporatesprogressively more sequence information. The likelihood of a particularoligonucleotide existing in such a distribution can be calculated asillustrated.

The selection of the particular oligonucleotide frequency distributionused to calculate probe likelihoods is essentially arbitrary. Asindicated above, to a certain extent, higher order distributions capturemore sequence information. The ability of a particular order probabilitydistribution to accurately reflect likelihood of occurrence of anoligonucleotide in a nucleic acid sample, can be determined by comparingpredicted likelihood values for various oligonucleotides with the actualfrequency of occurrence of those oligonucleotides in a sample nucleicacid data set. The closer the predicted likelihood approximates theactual likelihood, the better is the probability distribution.

Such a comparison can be made, for example, by dividing a collection ofsequences (e.g. the GenBank listing of human cDNAs) in half and usingthe first half to calculate a nucleic acid frequency distribution andpredict the likelihood of occurrence of particular oligonucleotides inthe second half of the collection. The predicted values can then becompared with the actual frequency of occurrence of the oligonucleotidesin the second half of the sequence set and the accuracy of theprediction determined.

iii) Selecting, Oligonucleotide Probes to Bind Unknown Sequences

The method described above provides a means of selecting probes expectedto bind to substantially all known sequences in a given nucleic acidsample. Conversely, it is possible to select oligonucleotide probesexpected to bind to N unknown nucleic acids in a sample, where N is thenumber of oligonucleotide probes to be selected. The selection methodbasically involves creating a list of all possible probes of a givenlength, deleting from the list probes that hybridize to known messages,and then selecting the N number of probes that have the greatestlikelihood of occurring in an unknown message. The method can beformalized as follows:

1) A list (L₁) of possible oligonucleotide probes (preferably allprobes) of length K is calculated;

2) Oligonucleotide probes from list L₁ that hybridize to known messages(or messages of interest) are deleted from the list.

3) The probability of occurrence of each of the oligonucleotide probesof list L₁ in a nucleic acid probability distribution is calculated;

4) The following steps are repeated until N probes are identified:

a) The list (L₁) is sorted from the highest probability to the lowest.

b) The highest probability probe is added to list of selected probes,L₂.

Step (4) can additionally include recalculating the probabilities on thecondition that the probability distribution contains no nucleic acidscomplementary to those oligonucleotides already selected.

The list of all possible probes of length K can be calculated in asimple combinatorial manner. Thus there are 4 probes of length 1 (a, C,G, T), 16 probes of length 2 (AA, AC, AG, AT, CA, CC . . . , TA, TC, TG,TT) and so forth. The distribution of message sequences can becalculated as described above as can the likelihood of occurrence ofeach probe in L₁ occurring in the distribution. As indicated above kmost preferably ranges from about 6 to about 50 nucleotides.

The probability distribution and likelihood is calculated as describedabove. The recalculation conditioned on the fact that no selected probe(L₂) is a complementary to an unknown is accomplished again, asdescribed above.

The limitations on thermal melting point can also be imposed on selectedprobes. Thus, only probes having a T_(m) above a critical thresholdand/or probes having a T_(m) within a specific allowable interval can beselected for inclusion in the affinity matrix.

Either of the above-described probe selection methods can include theadditional condition that the T_(m) of and by one of the selected probesto the other selected probes must be below the mismatch T_(m). This willprevent cross hybridization between probes comprising the affinitymatrix.

While the methods described above use nucleic acid samples (e.g.sequences selected from a genetic database such as GenBank) to predictthe nucleic acid frequency distribution of a “population” (e.g. theentire genome of a particular species), one of skill will appreciatethat where a particular genome is fully sequenced there is no need tocalculate the likelihood of occurrence of particular probes. Thosenucleic acid sequences that it is desired to capture with the affinitymatrix can be expressly included.

B) Ancillary Sequences to Facilitate Amplification/purification

As indicated above, the template nucleic acids forming the templatearray are used as templates for a nucleic acid amplification step (step3 of the method described above). In addition to the nucleic acidsequence (or subsequence) encoding the affinity ligands (capture probes)that are to be included in the affinity matrix, the nucleic acidtemplate(s) can additionally include ancillary sequences that facilitateamplification and/or purification.

For example, polymerase based amplification systems typically require adouble-stranded priming region for binding and initiation of thepolymerase. The double stranded priming region is typically provided byincluding, in the amplification reaction mixture, primers that arecomplementary to a portion of the template that is to be amplified. Theprimers bind to the template (at primer binding regions which arecomplementary to the primers) forming a double stranded region in towhich the polymerase binds and initiates transcription.

While primers can be selected that specifically hybridize to each of thetemplate nucleic acids such an approach may require a large number ofdifferent primers. Thus, in a preferred embodiment, the template nucleicacids additionally include terminal subsequences (primer bindingsequences) that are common to all of the template nucleic acids. All ofthe constituent nucleic acids can then be amplified with a single set ofprimers and one set of reaction conditions.

Thus, particularly preferred template nucleic acids comprise, in order,a common first priming domain, a “unique” domain that encodes theaffinity ligand (capture probe), and a second common priming domain(illustrated in FIG. 1, as Primer a* binding region, specific affinityligand, and Primer B* binding region, respectively). The first andsecond priming domains can be identical thereby allowing the use of asingle primer, or alternatively, the priming domains may have differentsequences and thus require two primers for amplification. Primingdomains can range in length from about 4 to about 50 nucleic acids,preferably from about 4 to about 40 nucleic acids, more preferably fromabout 4 to about 30 nucleic acids, and most preferably from about 4 toabout 20 nucleic acids. Particularly preferred priming domains can rangefrom about 6 to about 15 or about 20 nucleic acids in length. Primingdomains are preferably selected so that the priming domain does not alsoform a subsequence in any of the unique regions of the templates.

Where it is desired to remove the amplification priming domain from theamplification product, the primer domain can additionally incorporate anucleic acid sequence that is recognized by a restriction endonuclease.Thus, for example, a six nucleotide priming sequence can include a fourbase recognition site which when cleaved by a four base cutter leaves atwo base tail. Longer nucleotide priming sequences can provide longerrecognition sites (e.g. 5, 6, 7, . . . 10). One of skill will readilyappreciate that offset sites can be used allowing a clean cut end.Recognition sites and restriction endonucleases are well known to thoseof skill in the art and selection of particular recognition domains andtheir corresponding restriction enzyme are well known to those of skillin the art. (See, e.g. Gibco BRL, Grand Island, N.Y., USA, or NewEngland Biolabs, Beverly, Mass., USA for a list of restriction nucleasesand their recognition sites.)

On occasion, it will be desirable to purify one or more of the nucleicacids amplified from the nucleic acid template array. Such purificationcan be facilitated by incorporating specific (predetermined) nucleicacid “recognition” sequences into the nucleic acid template(s) of thenucleic acid sequences it is desired to purify. These nucleic acids canthen be isolated from the pool of amplified nucleic acids by use of anucleic acid affinity column with capture probes complementary to therecognition sequences. Thus, for example, a the template(s) can includea poly a tail which will facilitate purification of the amplificationproduct using an oligo dt column. Other recognition sequences are, ofcourse, suitable as well. Generally a recognition sequence will beselected such that it exists only in the amplified nucleic acids it isdesired to purify.

2) Preparation of a Template Array

While the nucleic acid templates can be provided free in solution, in apreferred embodiment the templates are themselves bound to a solidsupport forming an amplification template array. In a particularlypreferred embodiment, the amplification template arrays of thisinvention are high density oligonucleotide arrays comprising at leastabout 50, generally at least about 100, more generally at least about500, most generally at least about 1000, preferably at least about5,000, more preferably at least about 10,000, most preferably at leastabout 50,000, 100,000, 500,000 or even at least about 1,000,000different nucleic acid probes. Such high density arrays comprise a probedensity of generally greater than about 60, more generally greater thanabout 100, most generally greater than about 600, often greater thanabout 1000, more often greater than about 5,000, most often greater thanabout 10,000, preferably greater than about 40,000 more preferablygreater than about 100,000, and most preferably greater than about400,000 different oligonucleotide probes per cm². The capture probesincorporated into the affinity matrix, and hence the component of thenucleic acid template(s) encoding the affinity ligand (capture probe)itself can range in length from about 2 nucleotides acids up to about1000 nucleotides acids, generally from about 5 to about 500 nucleotidesacids, more generally from about 10 to about 150 nucleotides acids, mostgenerally from about 10 to about 100 nucleotides acids, preferably fromabout 10 to about 75 nucleotides acids, more preferably from about 10 toabout 50 nucleotides, and most preferably from about 15 to about 40nucleotides in length. Where the template nucleic acids includeancillary sequences, as discussed above, the template sequence lengthswill be increased accordingly.

Although a planar array surface is preferred, the array may befabricated on a surface of virtually any shape or even a multiplicity ofsurfaces. Methods of making such amplification template arrays arediscussed below.

A) Combinatorial Chemistry

As indicated above, the nucleic acids acting as “capture” probes in theaffinity matrices of this invention are preferably amplified from amultiplicity of amplificatian templates, more preferably from a group oftemplates that comprising a high density array of oligonucleotides.Methods of forming high density arrays of oligonucleotldes, peptides andother polymer sequences with a minimal number of synthetic steps areknown. The oliganucleotide analogue way can be synthesized on a solidsubstrate by a variety of methods, including, but not limited to,light-directed chemical coupling, and mechanically directed coupling.See Pirrung et al., U.S. Pat. No. 5,143,854 (see also PCT ApplicationNo. WO 90/15070) and Fodor et al., PCT Publication Nos. WO 92/10092 andWO 93/09668 which disclose methods of forming vast arrays of peptides.oligonucleotides and other molecules using, for example, light-directedsynthesis techniques, which are incorporated herein by reference. Seealso, Fodor et al., Science, 251, 767-77 (1991) which is incorporatedherein by reference. These procedures for synthesis of polymer arraysare now referred to as VLSIPS™ procedures. Using the VLSIPS™ approach,one heterogenous array of polymers is converted, through simultaneouscoupling at a number of reaction sites, into a different heterogenousarray. See, U.S. application Ser. Nos. 07/796,243. now U.S. Pat. No.5,384,261 and 07/980,523, now U.S. Pat. No. 5,677,195, which areincorporated herein by reference.

The development of VLSIPS™ technology as described in the above-notedU.S. Pat. No. 5,343,854 and PCT patent publication Nos. WO 90/15070 and92/10092, currently in national states. is considered pioneeringtechnology in the fields of combinatorial synthesis and screening ofcombinatorial libraries. U.S. patent application Ser. No. 08/082,937(currently abandoned), filed Jun. 25, 1993 (and incorporated herein byreference) describes methods for making arrays of oligonucleotide probesthat can he used to check or determine a partial or complete sequence ofa target nucleic acid and to detect the presence of a nucleic acidcontaining a specific oligonucleotide sequence.

In brief, the light-directed combinatorial synthesis of oligonucleotidearrays on a glass or other surface proceeds using automatedphosphoramidite chemistry and chip masking techniques. The one specificimplementation, a glass surface is derivatized with a silane reagentcontaining a functional group, e.g., a hydroxyl or amine group blockedby a photolabile protecting group. Photolysis through a photolithogaphicmask is used selectively to expose functional groups which are thenready to react with incoming 5′-photoprotected nucleosidephosphoramidites. The phosphoramidites react only with those sites whichare illuminated (and thus exposed by removal of the photolabile blockinggroup). Thus, the phosphoramidites only add to those areas selectivelyexposed from the preceding step. These steps are repeated until thedesired array of sequences have been synthesized on the solid surface.Combinatorial synthesis of different oligonucleotide analogues atdifferent locations on the array is determined by the pattern ofillumination during synthesis and the order of addition of couplingreagents.

In the event that an oligonucleotide analogue with a polyamide backboneis used in the VLSIPS™ procedure, it is generally inappropriate to usephosphoramidite chemistry to perform the synthetic steps, since themonomers do not attach to one another via a phosphate linkage. Instead,peptide synthetic methods are substituted. See, e.g., Pirrung et al.U.S. Pat. No. 5,143,854 which is incorporated herein by reference.

Peptide nucleic acids are commercially available from, e.g., Biosearch,Inc. (Bedford, Mass.) which comprise a polyamide backbone and the basesfound in naturally occurring nucleosides. Peptide nucleic acids arecapable of binding to nucleic acids with high specificity, and areconsidered “oligonucleotide analogues” for purposes of this disclosure.

In addition to the foregoing, additional methods which can be used togenerate an array of oligonucleotides on a single substrate aredescribed in co-pending U.S. patent application Ser. Nos. 07/980,523,U.S. Pat. No. 5,677,195, filed Nov. 20, 1992, and 07/796,243, now U.S.Pat. No. 5,384,261, filed Nov. 22, 1991 and in PCT Publication No. WO93/09668, which are incorporated herein by reference. In the methodsdisclosed in these applications, reagents are delivered to the substrateby either (1) flowing within a channel defined on predefinad regions or(2) “spotting” on predefined regions. However, other approaches, as wellas combinations of spotting and flowing, maybe employed. In eachinstance, certain activated regions of the substrate are mechanicallyseparated from other regions when the monomer solutions are delivered tothe vaious reaction sites.

A typical “flow channel” method applied to the compounds and librariesof the present invention can generally be described as follows. Diversepolymer sequences are synthesized at selected regions of a substrate orsolid support by forming flow channels on a surface of the substratethrough which appropriate reagents flow or in which appropriate reagentsare placed. For example, assume a monomer “a” is to be bound to thesubstrate in a first group of selected regions. If necessary, all orpart of the surface of the substrate in all or a part of the selectedregions is activated for binding by, for example, flowing appropriatereagents through all or some of the channels, or by washing the entiresubstrate with appropriate reagents. After placement of a channel blockon the surface of the substrate, a reagent having the monomer a flowsthrough or is placed in all or some of the channel(s). The channelsprovide fluid contact to the first selected regions, thereby binding themonomer a on the substrate directly or indirectly (via a spacer) in thefirst selected regions.

Thereafter, a monomer B is coupled to second selected regions, some ofwhich may be included among the first selected regions. The secondselected regions will be in fluid contact with a second flow channel(s)through translation, rotation, or replacement of the channel block onthe surface of the substrate; through opening or closing a selectedvalve; or through deposition of a layer of chemical or photoresist. Ifnecessary, a step is performed for activating at least the secondregions. Thereafter, the monomer B is flowed through or placed in thesecond flow channel(s), binding monomer B at the second selectedlocations. In this particular example, the resulting sequences bound tothe substrate at this stage of processing will be, for example, a, B,and AB. The process is repeated to form a vast array of sequences ofdesired length at known locations on the substrate.

After the substrate is activated, monomer a can be flowed through someof the channels, monomer B can be flowed through other channels, amonomer C can be flowed through still other channels, etc. In thismanner, many or all of the reaction regions are reacted with a monomerbefore the channel block must be moved or the substrate must be washedand/or reactivated. By making use of many or all of the availablereaction regions simultaneously, the number of washing and activationsteps can be minimized.

One of skill in the art will recognize that there are alternativemethods of forming channels or otherwise protecting a portion of thesurface of the substrate. For example, according to some embodiments, aprotective coating such as a hydrophilic or hydrophobic coating(depending upon the nature of the solvent) is utilized over portions ofthe substrate to be protected, sometimes in combination with materialsthat facilitate wetting by the reactant solution in other regions. Inthis manner, the flowing solutions are further prevented from passingoutside of their designated flow paths.

The “spotting” methods of preparing compounds and libraries of thepresent invention can be implemented in much the same manner as the flowchannel methods. For example, a monomer a can be delivered to andcoupled with a first group of reaction regions which have beenappropriately activated. Thereafter, a monomer B can be delivered to andreacted with a second group of activated reaction regions. Unlike theflow channel embodiments described above, reactants are delivered bydirectly depositing (rather than flowing) relatively small quantities ofthem in selected regions. In some steps, of course, the entire substratesurface can be sprayed or otherwise coated with a solution. In preferredembodiments, a dispenser moves from region to region, depositing only asmuch monomer as necessary at each stop. Typical dispensers include amicropipette to deliver the monomer solution to the substrate and arobotic system to control the position of the micropipette with respectto the substrate. In other embodiments, the dispenser includes a seriesof tubes, a manifold, an array of pipettes, or the like so that variousreagents can be delivered to the reaction regions simultaneously.

B) Combined Synthetic Methods

Using the light-directed coupling and mechanically-directed couplingmethods described above, one can readily synthesize high densityamplification template arrays. However, where all of the templatenucleic acids include common sequences (e.g. the primer binding regions)the high density amplification template arrays can be produced usingcombined (hybrid) synthesis methods. Combined methods utilize standard(e.g. DMT protected phosphoramidite chemistry) for synthesis of thecommon regions of the templates and light-directed ormechanically-directed methods for the specific binding (affinity ligand)regions of the templates. Thus, for example, the first primer bindingregions can be synthesized on the solid support according to anystandard oligonucleotide synthesis method (e.g. standard phosphotriesterchemistry). The final base added can bear a MenPoc or an Fmoc protectinggroup allowing the “unique” (affinity ligand) region to be synthesizedas described above using any of the light-directed ormechanically-directed coupling methods. The last nucleotide added to the“unique” region preferably bears a terminal dimethoxytrityl (DMT)protecting group thereby allowing the second primer binding region to besynthesized using standard oligonucleotide synthesis chemistry.

While any oligonucleotide synthesis method, including solid and liquidphase, phosphite-triester and phosphotriester chemistries, is suitable,oligonucleotide synthesis is preferably carried in solid phase using thesolid phase phosphoramidite triester method described by Beaucage et.al., Tetrahedron Letts. 22 (20): 1859-1862 (1981). The synthesis can beperformed manually or using an automated oligonucleotide synthesizer(see, e.g. Needham-VanDevanter et al. Nucleic Acids Res. 12:6159-6168(1984)). Methods of oligonucleotide synthesis are routine and well knownto those of skill in the art. (See, e.g. Itakura, U.S. Pat. No.4,401,796; Caruthers et al., U.S. Pat. Nos. 4,458,066 and 4,500,707;Beaucage et al., Tetrahedron Lett., 22: 1859-1862 (1981); Matteucci etal., J. Amer. Chem. Soc., 103: 3185-3191 (1981); Caruthers et al.,Genetic Engineering, 4: 1-17 (1982); Jones, chapter 2, Atkinson et al.,chapter 3, and Sproat et al., chapter 4, in Gait, ed. OligonucleotideSynthesis: a Practical Approach, IRL Press, Washington D.C. (1984);Froehler et al., Tetrahedron Lett., 27: 469-472 (1986); Froehler et al.,Nucleic Acids Res., 14: 5399-5407 (1986); Sinha et al. TetrahedronLett., 24: 5843-5846 (1983); and Sinha et al., Nucl. Acids Res., 12:4539-4557 (1984).

One of skill in the art will appreciate that where different synthesisare to be combined for template synthesis, different protecting groupsshould also be incorporated. Thus, for example, where the first primerbinding domain is synthesized using a phosphotriester chemistry, the 5′hydroxyl group is preferably protected with a dimethoxytrityl (DMT)group. The last nucleotide coupled however, is preferably protected witha protecting group compatible with the light-directed ormechanically-directed synthesis methods (e.g. a Fmoc, Tboc, or MenPoc)with a MenPoc group being most preferred for compatibility withlight-directed coupling methods. Then the last nucleotide of the uniqueregion added using light-directed synthesis methods will be protectedwith a DMT group (or other group compatible with the new synthesischemistry) thereby facilitating synthesis of the second primer bindingregion using phosphotriester chemistry again.

The common primer binding regions of the template oligonucleotides canbe synthesized de novo as described above using the light-directed,mechanically directed or hybrid synthesis methods. Alternatively, thecomplete primer binding regions can be synthesized in a batch processand the completed primer binding region sequence can then be coupled tothe solid support forming the amplification template array or to theunique (affinity ligand) region of the templates present in the templatearray. Synthesis according to this approach thus involves coupling thefirst primer binding region to the substrate of the amplificationtemplate array (either directly or through a linker). The primer bindingregion can be provided with a terminal protecting group, or one can beadded afterwards. The unique region is then synthesized usinglightdirected or mechanically-directed coupling methods. Finally thesecond complete primer binding region is covalently linked to theterminus of the unique region. Coupling of oligonucleotides simplyinvolves linking the 3′ hydroxyl of one oligonucleotide with the 5′hydroxyl of a second oligonucleotide (or to an activated site on a solidsupport or linker). The linkage can be through the formation of aphosphodiester linkage. Typically this is accomplished by providing oneof the oligonucleotides with an activated or activatable terminalphosphate. Methods of providing such phosphate groups and linkingoligonucleotides are well known to those of skill in the art. (See,e.g., WO 85/01051 and WO/90/03382 which are incorporated herein byreference).

Both the above-described combinatorial synthetic methods as well as thestandard synthetic methods result in the production of oligonuleotideslinked to a solid support (e.g. a glass slide or controlled pore glass)via a linker. Typically in these methods oligonucleotide synthesiscommences by coupling of a nucleotide to a reactive group on a linkerwhich in turn is bound to the solid support. The reactive group can be aterminal hydroxyl directly on the solid support, or the 3′ or 5′hydroxyl of a nucleotide which in turn is bound to the solid supporteither directly or through a linker. Suitable linkers are well known tothose of skill in the art. (see, e.g. Gait, et al. ed. OligonucleotideSynthesis: a Practical Approach, IRL Press, Washington D.C. (1984); U.S.Pat. No. 5,143,854; PCT Application No. WO 90/15070; Fodor et al., PCTPublication Nos. WO 92/10092 and WO 93/09668, and copending U.S. Ser.No. 08/431,196, now U.S. Pat. No. 5,624,711, filed on Apr. 27, 1995; and08/374,492, now U.S. Pat. No. 5,679,173 filed on Jan. 17, 1995).

Where it is desired to perform the subsequent amplification with thetemplate nucleic acids in solution phase, the linkers can be cleavedaccording to standard methods (e.g. treatment with concentratedammonia). Conversely, where the subsequent amplifications are to beperformed with the template nucleic acids attached to a solid support,the linker is not cleaved before amplification. In a preferredembodiment, the amplification is performed with the templateoligonucleotides retained on the solid support. The amplificationtemplate array can then be reused as a template for a number ofamplifications and the production of a large number of affinitymatrixes.

3) Amplification of the Nucleic Acid Templates to Provide a Populationof Capture Probes

a) Amplification

The affinity matrices of this invention comprise at least about 50,preferably at least about 100, more preferably at least about 500, 1000,5,000, 10,000, 50,000, 100,000, 500,000 and even at least about1,000,000 different nucleic acid probes. One of skill in the art willappreciate that the chemical synthesis of each nucleic acid individuallyin quantities sufficient to provide an affinity matrix with sufficientloading capacity to isolate useful quantities of target nucleic acids,is an expensive and time-consuming task.

Thus, in a preferred embodiment, the nucleic acid probes are obtained byamplification of the template nucleic acids in the above-describedtemplate array. The template pool includes at least one nucleic acidthat provides the amplification template for each of the nucleic acidsequences that are to be included in the nucleic acid affinity matrix.Thus, template arrays can include least about 50, preferably at leastabout 100, more preferably at least about 500, 1000, 5,000, 10,000,50,000, 100,000, 500,000 and even at least about 1,000,000 differentnucleic acid templates.

The nucleic acids can be amplified by any of the amplification methodswell known in the art, which include, but are not limited to polymerasechain reaction (PCR) (Innis, et al., PCR Protocols. a guide to Methodsand Application. Academic Press, Inc. San Diego, (1990)), ligase chainreaction (LCR) (see Wu and Wallace, Genomics, 4: 560 (1989), Landegren,et al., Science, 241: 1077 (1988) and Barringer, et al., Gene, 89: 117(1990)), transcription amplification (see Kwoh, et al., Proc. Natl.Acad. Sci. (U.S.A.), 86: 1173 (1989)), and self-sustained sequencereplication (see Guatelli, et al., Proc. Nat. Acad. Sci. (U.S.A.), 87:1874 (1990)).

Amplification of the nucleic acids comprising the template pool can beperformed in solution or with the template nucleic acids anchored to asolid support (e.g., a glass slide) and thereby forming a templatearray. In a preferred embodiment, amplification is by polymerase chainreaction (PCR). PCR amplification methods are well known to those ofskill in the art. Basic amplification conditions can be found in a widevariety of references such as PCR Protocols: a Guide to Methods andApplications, Innis et al., eds. Academic Press, Inc. New York (1990).

One of skill will appreciate that amplification can be optimized forparticular primer/template array combinations. Optimization typicallyinvolves adjusting magnesium ion concentration, buffer composition (e.g.KCl and NaCl ion concentration and buffer pH), and the temperature andlength of time allowed for primer annealing, extension and denaturation.Methods of optimizing amplification protocols are routine and well knownto those of skill in the art (see, e.g. Chapter 1 in PCR Protocols: aGuide to Methods and Applications, Innis et al., eds. Academic Press,Inc. New York (1990)). In a standard and suitable PCR protocol, thereaction mix includes about 20 pmol of each primer (T_(m)>55° C.preferred), 20 mM Tris-HCl (pH 8.3) (20° C.), 1.5 mM MgCl₂, 25 mM KCl,0.05% Tween 20, 100 μg/ml autoclaved gelatin or nuclease-free bovineserum albumin, 50 μM each dNTP, and 2 units of Taq DNA polymerase.

The amplification is typically run through enough cycles to provide thedesired amount of amplified product (capture probe). In a preferredembodiment, the amplification will be cycled enough to produce at leasta 2 fold, generally at least a 5 fold, more generally at least a 10fold, preferably at least a 100 fold, more preferably at least a 1000fold, and most preferably at least a 10,000 fold amplification. Wheremost of the amplified nucleic acid pool is incorporated into theaffinity matrix, the matrix will thus contain at least a 2 fold,generally at least a 5 fold, more generally at least a 10 fold,preferably at least a 100 fold, more preferably at least a 1000 fold,and most preferably at least a 10,000 fold more nucleic acid moleculesof each species than the original nucleic acid template array.

A standard amplification will involve 20 to 40 cycles, more preferablyabout 25 to 35 cycles. In a preferred embodiment cycling is carried outfor about 25 to about 35 cycles using the following temperature profile:

Denaturation: about 96° C., 15 seconds (a longer initial time is usuallydesirable); Primer annealing; about 55° C., 30 seconds; and PrimerExtension: about 72° C., 1.5 minutes.

Cycling preferably concludes with a final extension at about 72° C. for5 minutes. Reactions can be stopped by chilling to 4° C. and/or byaddition of EDTA to 10 mM.

B) Post Amplification Processing

While the amplified nucleic acid pool can be used directly, in apreferred embodiment, the amplified nucleic acids are purified away fromreaction components (e.g. incomplete amplification products, nucleotidetriphosphates, etc.). Means of purifying nucleic acid amplificationproducts are well known to those of skill in the art. Typically theamplification products are the largest nucleic acids in the reactionmixture and purification techniques based on molecule size are highlyeffective. Such methods include, but are not limited to gelelectrophoresis, high performance liquid chromatography (HPLC), affinitychromatography (e.g. using an affinity column complementary to theprimer binding region), capillary electrophoresis, density gradientcentrifugation, and the like. Particularly preferred is HPLC using ananion exchange column (e.g. mono Q from Pharmacia, Piscataway, N.J.).

Where the amplified nucleic acids include restriction sites (e.g., forremoval of the primer regions) and the amplified nucleic acids are to becleaved at the reaction sites before use the cleavage reaction can beperformed before or after purification. However, in a preferredembodiment, it is desirable to cleave the amplification products priorto purification so that the desired cleaved product (e.g., captureprobes) are purified away from the undesired sequences (e.g., primerbinding regions) in the single purification step described above.

Cleavage of the amplified nucleic acids containing restriction sites isaccomplished according to standard methods well known to those of skillin the art. Typically, the amplification product is combined with therestriction endonuclease specific for the cleavage site under conditions(e.g., temperature, pH) where the restriction endonuclease is active.Suitable reaction conditions for each particular restrictionendonuclease are provided by the supplier or manufacturer.

3) Attachment of the Amplified Nucleic Acids to a Solid Support toProduce an Affinity Matrix

The affinity matrices of this invention can be formulated with virtuallyany solid material or gel that does not substantially interfere withhybridization of the oligonucleotides. Suitable matrix materials includepaper, glasses, ceramics, metals, metalloids, polacryloylmorpholide,various plastics and plastic copolymers such as Nylons™, Teflon™,polyethylene, polypropylene, poly(4-methylbutene), polystyrene,polystyrene, polystyrene/latex, polymethacrylate, poly(ethyleneterephthalate), rayon, nylon, poly(vinyl butyrate), polyvinylidenedifluoride (PVDF), silicones, polyformaldehyde, cellulose, celluloseacetate, nitrocellulose, and the like. Materials that typically bindnucleic acids (e.g. cellulose) are suitable, however, in a preferredembodiment, an affinity matrix composed of such materials is preferablyprehybridized with a blocking nucleic acid (e.g. sperm DNA or C₀t−1 DNA)to reduce non-specific binding.

The affinity matrix can be loaded with virtually any amount ofoligonucleotide affinity ligand, the loading being only limited byavailable binding sites for attachment of the ligand and thus, onlylimited by the available amount of solid or gel support.

The affinity matrix can take any form that is convenient includingbeads, porous beads, crushed particles, membranes, tubing, planarsurfaces, etc. Preferred matrix materials are particulate (e.g. beads)thereby providing increased surface area for attachment of affinityligands. Particularly preferred matrix materials can be porous(fenestrated) highly convoluted and/or rugose (e.g. controlled poreglass, folded membranes, etc.).

Methods of attaching oligonucleotides to solid supports (matrixmaterials) are well known to those of skill in the art. For example, ina preferred embodiment, the primers used in the amplification can beprovided with a conjugated biotin or streptavidin. The amplified nucleicacids will then bear the biotin or streptavidin and can be coupled to asolid support bearing avidin (streptavidin) or biotin respectively.

Alternatively, the nucleic acid can be covalently coupled to the solidsupport either directly via an activated group (e.g. a hydroxyl, acarboxyl) or through a linker that provides reactive moieties that bindto the oligonucleotide and to the matrix material respectively. Linkerssuitable for attaching nucleic acids to matrix materials are also wellknown. Generally linkers are either hetero- or homo-bifunctionalmolecules that contain two or more reactive sites that may each form acovalent bond with the respective binding partner (the matrix materialor the nucleic acid). For example, the probe oligonucleotides may bejoined by a peptide linker, by a straight or branched chain carbon chainlinker, or by a heterocyclic carbon. Heterobifunctional cross linkingreagents such as active esters of N-ethylmaleimide have been widelyused. See, for example, Lerner et al. Proc. Nat. Acad. Sci. (USA), 78:3403-3407 (1981) and Kitagawa et al. J. Biochem., 79: 233-236 (1976).Other linkers, such as those used in the synthesis of nucleic acids arealso suitable (see, e.g. PCT Publication WO 85/01051, Pochet et al.Tetrahedron. 43: 3481-3490 (1987), Schwyzer et al., Helv. Chim. Acta,67: 1316-1327 (1984), Gait, ed. Oligonucleotide Synthesis: a PracticalApproach, IRL Press, Washington D.C. (1984)).

As indicated above, any material to which the oligonucleotide can bebound and which is resistant to nucleic acid hybridization reagents(e.g. Tris-HCl, SSC, etc.) and temperatures (e.g. 30° C. to 80° C.) anddoes not substantially interfere with the oligonucleotide hybridizationis suitable for use as a matrix material. Particularly preferred matrixmaterials include glass beads, controlled pore glass, and variouspolymeric resins such as polystyrene, polystyrene/latex, and the like.

In a preferred embodiment, the amplified nucleic acids are purified awayfrom the other components of the amplification mixture (e.g.triphosphates, truncated amplification products, etc.) prior toattachment to the matrix material. Methods of purifying nucleic acidsare well known to those of skill in the art and described above insection 3.

IV. Hybridization Conditions

The affinity matrices of this invention rely on hybridization betweenthe nucleic acids comprising the affinity matrix and any target nucleicacids that may be present in the sample to specifically bind to andremove the target nucleic acids from the sample. Nucleic acidhybridization simply involves contacting the oligonucleotide probes ofthe affinity matrix and the target nucleic acid under conditions wherethe probe and its complementary target can form stable hybrid duplexesthrough complementary base pairing. The nucleic acids that do not formhybrid duplexes are then washed away leaving the hybridized nucleicacids bound to the affinity matrix. Where the bound nucleic acids are tobe retrieved, the duplexes can be denatured (e.g. by increasingtemperature, adding formamide or decreasing salt) and the freed nucleicacids recovered. Alternatively the sample contacted with the affinitymatrix can be recovered and will be lacking those nucleic acids capturedand removed by the affinity matrix.

It is generally recognized that nucleic acids are denatured byincreasing the temperature or decreasing the salt concentration of thebuffer containing the nucleic acids. Under low stringency conditions(e.g., low temperature and/or high salt) hybrid duplexes (e.g., DNA:DNA,RNA:RNA, or RNA:DNA) will form even where the annealed sequences are notperfectly complementary. Thus specificity of hybridization is reduced atlower stringency. Conversely, at higher stringency (e.g., highertemperature or lower salt) successful hybridization requires fewermismatches.

One of skill in the art will appreciate that hybridization conditionsmay be selected to provide any degree of stringency. In a preferredembodiment, hybridization is performed at low stringency in this case in6×SSPE-T at 37° C. (0.005% Triton X-100) to ensure hybridization andthen subsequent washes are performed at higher stringency (e.g.,1×SSPE-T at 37° C.) to eliminate mismatched hybrid duplexes. Successivewashes may be performed at increasingly higher stringency (e.g., down toas low as 0.25×SSPE-T at 37° C. to 50° C.) until a desired level ofhybridization specificity is obtained. Stringency can also be increasedby addition of agents such as formamide. Hybridization specificity maybe evaluated by comparison of hybridization to the sample nucleic acidswith hybridization to the various controls that can be present (e.g.,expression level control such as mRNAs spiked into the sample at knownconcentrations, etc.).

In general, there is a tradeoff between hybridization specificity(stringency) and signal intensity. Thus, in a preferred embodiment, thewash is performed at the highest stringency that produces consistentresults or achieves maximum specific removal of the target nucleic acidsfrom the sample.

The stability of duplexes formed between RNAs or DNAs are generally inthe order of RNA:RNA>RNA:DNA>DNA:DNA, in solution. Long probes havebetter duplex stability with a target, but poorer mismatchdiscrimination than shorter probes (mismatch discrimination refers tothe measured hybridization signal ratio between a perfect match probeand a single base mismatch probe). Shorter probes (e.g., 8-mers)discriminate mismatches very well, but the overall duplex stability islow.

Altering the thermal stability (T_(m)) of the duplex formed between thetarget and the probe using, e.g., known oligonucleotide analogues allowsfor optimization of duplex stability and mismatch discrimination. Oneuseful aspect of altering the T_(m) arises from the fact thatadenine-thymine (a-T) duplexes have a lower T_(m) than guanine-cytosine(G-C) duplexes, due in part to the fact that the a-T duplexes have 2hydrogen bonds per base-pair, while the G-C duplexes have 3 hydrogenbonds per base pair. In heterogeneous oligonucleotide matrices in whichthere is a non-uniform distribution of bases, it is not generallypossible to optimize hybridization for each oligonucleotide probesimultaneously. Thus, in some embodiments, it is desirable toselectively destabilize G-C duplexes and/or to increase the stability ofa-T duplexes. This can be accomplished, e.g., by substituting guanineresidues in the probes of a matrix which form G-C duplexes withhypoxanthine, or by substituting adenine residues in probes which forma-T duplexes with 2,6 diaminopurine or by using the salt tetramethylammonium chloride (TMACl) in place of NaCl.

Altered duplex stability conferred by using oligonucleotide analogueprobes can be ascertained by following, e.g., fluorescence signalintensity of oligonucleotide analogue arrays hybridized with a targetoligonucleotide over time. The data allow optimization of specifichybridization conditions at, e.g., room temperature (for simplifieddiagnostic applications in the future).

Another way of verifying altered duplex stability is by following thesignal intensity generated upon hybridization with a labeled sample withtime. Previous experiments using DNA targets and DNA chips have shownthat signal intensity increases with time, and that the more stableduplexes generate higher signal intensities faster than less stableduplexes. The signals reach a plateau or “saturate” after a certainamount of time due to all of the binding sites becoming occupied. Thesedata allow for optimization of hybridization, and determination of thebest conditions at a specified temperature.

Methods of optimizing hybridization conditions are well known to thoseof skill in the art (see, e.g., Laboratory Techniques in Biochemistryand Molecular Biology, Vol. 24: Hybridization With Nucleic Acid Probes,P. Tijssen, ed. Elsevier, N.Y., (1993)).

In a preferred embodiment, the affinity matrix is packed into a columnarcasing. The sample is then applied to the affmiity matrix (e.g. injectedonto a column or applied to a column by a pump such as a sampling pumpdriven by an autosampler). The affinity matrix (e.g. affinity column)bearing the sample is subjected to conditions under which the nucleicacid probes comprising the affinity matrix hybridize specifically withcomplementary target nucleic acids. Such conditions are accomplished bymaintaining appropriate pH, salt and temperature conditions tofacilitate hybridization as discussed above.

It is understood that the embodiments described herein are forillustrative purposes only and that various modifications or changes inlight thereof will be suggested to persons skilled in the art and are tobe included within the spirit and purview of this application and scopeof the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference in theirentirety for all purposes.

What is claimed is:
 1. A method for eliminating a plurality ofpreselected gene products from a nucleic acid sample comprising:providing an affinity matrix comprising at least one nucleic acidaffinity ligand complementary to each of the plurality of preselectedgene products; hybridizing the nucleic acid sample to the affinitymatrix; and eluting the sample from the affinity matrix; therebysubstantially depleting the nucleic acid sample of the preselected geneproducts.
 2. The method of claim 1 wherein the nucleic acid affinityligand is an oligonucleotide probe.
 3. The method of claim 2 wherein thenucleic acid affinity ligand is attached to a solid support.
 4. Themethod of claim 3 wherein the solid support is selected from the groupconsisting of beads, porous beads, crushed particles, membranes, tubingand planar surfaces.
 5. The method of claim 4 wherein the solid supportis particulate.
 6. The method of claim 1 wherein, at least, one of thepreselected gene products is actin.
 7. The method of claim 1 wherein, atleast, one of the preselected gene products is GAPDH.